The present invention is directed to a method for manufacturing preforms for polymer optical fibers having a variable refractive index over the radius.
Polymer optical fibers (POF or plastic fibers) are optical waveguides which are apparently being used to an increasing degree to transmit information and data.
When setting up broadband communications networks, the terminal branches between the principal conductors of the network and the end customers may account by far for the greatest share of required data lines. Therefore, it may be economically significant to reduce the costs of the networks"" terminal branches.
It is believed that, under the state of the art, metal or fiber optic cables are still predominantly employed for the terminal branches of modern fiber optic networks.
Since, for the most part, terminal branches turn out to be quite short in length, it suffices, however, to use plastic fibers which do, in fact, exhibit a higher attenuation than optical glass fibers, but are much simpler to process (or machine). Moreover, the requisite plastics may be manufactured in virtually any desired quantity at low costs, which may also constitute an advantage over metal cables. In addition, they are insensitive to induced interference voltage and possess a high transmission (or communication) capacity.
At the present time, virtually only so-called step-index POFs are available. These are optical waveguides having a core of a plastic of a higher refractive index and a plastic cladding of a lower refractive index. This type of fiber has a typical transmission bandwidth of 100 Mb/s over 100 m length. This value does not suffice for broadband networks. They require transmission rates in the Gb/s range.
Such high transmission bandwidths may be achieved using graded index POF (GI-POF). These are fibers whose refractive index in the core may decrease steadily from the axis toward the edge more or less in a parabolic curve shape. However, it is believed that there is no prior economical manufacturing process for GI-POF based on any methods that may be available. In this context, a main problem is being able to produce large enough preforms for the fiber manufacturing.
An exemplary method and/or exemplary embodiment of the present invention is directed to providing a method for economically producing preforms having adjustable refractive index profiles.
Another exemplary method and/or exemplary embodiment of the present invention is directed to providing that a core of preform material for forming the preform is coated with coating material until a predefinable diameter is reached, the refractive index of the coating material to be applied being successively varied in conformance with a predefinable profile along the radius of the emerging preform. Provision may also be made in the manufacturing of GI-POFs for the refractive index of the coating material to be reduced along the radius of the preform. Through this method, the refractive index of the material may be adjusted virtually infinitely, continuously and/or steplessly over the radius of the preform, in that prior to or during the coating operation. Measures may be taken to selectively vary the refractive index of the coating material to be applied.
Another exemplary method and/or exemplary embodiment of the present invention is directed to providing for the coating material to be sputtered (or injection molded) or extrusion-coated on, material having different refractive indices being added to varying degrees to the coating material to vary the refractive index, or dopants being added to varying degrees to the coating material to vary the refractive index.
Another exemplary method and/or exemplary embodiment of the present invention is directed to providing for at least one film of the coating material to be coiled around the core, while continuously fusing with the core material, until a predefinable diameter is reached, the refractive index of the film being varied with the length; provision may be made for the film to be fused on using a device which is situated alongside the core and which emits a radiation such that the radiation may be absorbed by the coiled-on film under heating, thereby leading to softening and fusing of the film.
It is believed that by alternating films having different indices of refraction, by pretreating the films to be coiled on, for example, by irradiating them or by adding dopants thereto, the refractive indices during the coiling can be precisely adjusted. In this manner, preforms having virtually any desired thickness and whose refractive index profile is precisely adjusted, may be manufactured economically. The slightly graded (or stepped) nature of the preform""s refractive index profile may be smoothed by scaling effects and by the diffusion (or inward diffusion) of dopants when the fiber is fabricated from the preform, so that the fiber exhibits a virtually constant refractive index profile over the radius.